The invention relates to an electroacoustic transducer of the piezoceramic type, which is suitable for use as a piezoceramic capsule or as a tone caller in telephone instruments.
Piezoceramic transducers currently used in telephone instruments comprise essentially a transducer diaphragm, maintained peripherally in position in a cavity between a bottom of the case, with one wall perforated in order to allow the transmission of sound, and a cover to which terminals for external electrical connection are applied.
The electrical connection between the diaphragm and these terminals is established either by means of thin wires or extensions of the terminals themselves both soldered on the poles of the diaphragm.
Soldering on the diaphragm causes drawbacks. In fact, the solder joints on the ceramic layer often detach; moreover, efficiency problems may arise since the solderings are not perfectly equal in all the pieces of a production series and the efficiency is not constant; last, but not least, costs are high. In order to avoid soldering on the diaphragm several attempts have been made.
From French patent application no. 8806644, there is known a telephone transducer with a piezoelectric unit in which the electrical connections to the diaphragm are established through two spring contact terminals of dutch metal bronze which are arranged in such a way as to apply pressure, one on the annular portion of the diaphragm substrate, and one on the ceramic layer covering the central portion of the substrate.
With structures like that soldering of the diaphragm is unnecessary; however, they are not free of drawbacks.
First of all, the electrical connection members manufactured by punching and bending from a matrix of rolled material, do not allow a constant contact pressure on the diaphragm because of manufacturing tolerances, and this create problems of efficiency for the diaphragm.
Moreover, the bends could change their angulation during the assembling of the unit; the springs require adjusting and, consequently, the connection members which cannot be forced into the terminal fitting holes of the bottom require a successive sealing of such holes in order to avoid leakage of the unit.
Further, the spring contact on the ceramics applies pressure on the inner area of the diaphragm and this worsens the situation.
From Patent Abstracts of Japan, vol. 10, no. 46 (E-383) (2103), Feb. 22, 1986, there is known a piezoelectric acoustic transducer in which a piezoelectric vibrator is constituted of a unimorphic vibrator where a piezoelectric ceramic element is bonded to a metallic plate and arranged so that the side of the element will point the opening side of a case. A support ring is formed by an insulating part and a conductive part and a spring terminal is fitted to a notch. The terminal consists of a contact part and a fitting part formed with a spring material and temporarily fixed after a folding piece of the fitting part is fitted in a not-notched thin part of the insulating part. A damping film is applied to a copper foil patterns of a cover. External terminals are inserted into terminal fitting holes of the cover and connected to the copper foils respectively.
Also with this solution soldering on the diaphragm is unnecessary; nevertheless, it is still not free of drawbacks since it uses spring terminals. Moreover, the contact between the ceramics and the corresponding terminals is established at the inner zone of the diaphragm which is not allowed to vibrate freely.